1. Field of the Invention
The invention relates to solid state electrochemical sensors. More particularly, the invention relates to electrochemical sensors which are submersed in a solution, such as pH sensors and the like.
2. State of the Art
In the late 1960s, Dutch researcher Piet Bergveld investigated the pH sensitivity of an electronic device known as an ion selective field effect transistor (ISFET). ISFETs are similar to conventional insulated gate FETs (field effect transistors) except that the metal gate electrode is removed and the gate region is covered with an insulator film. The film acts as an ion selective, sensitive membrane which affects the conductance between the drain and the source terminals of the transistor. Depending on the material chosen for the membrane, the membrane will react to different kinds of ionic activity. When the membrane is made of silicon nitride or aluminum oxide, it will react to hydrogen ions and the device can be used to measure pH.
In order for an ISFET to be useful in measuring the pH of a solution, it must be submersed in the solution. This raises many design problems since the solution is most often hostile to the ISFET circuit. It is therefore desirable to isolate the membrane from the other parts of the ISFET circuit so that the membrane may be in contact with the solution while the remainder of the ISFET circuit is protected from the adverse affects of the solution.
U.S. Pat. No. 4,505,799, the complete disclosure of which is hereby incorporated by reference herein, discloses an ISFET sensor and a method of manufacturing the sensor. The ISFET sensor 10, which is shown in prior art FIG. 1, includes an N-Si silicon crystal substrate 12 having an orientation of (100) which is coated on the top surface with a number of oxides 14 and an ion sensitive membrane 16. The substrate 12 is doped to have a P+ drain region 18 and a P+ source region 20. A window 21 is formed in the field oxide 13 in the area between the source and drain where the gate oxide 22 has been grown under the ion sensitive membrane 16. On the bottom surface of the substrate 12, windows 25, 27, and 29 are formed in the field oxide coating 15. Window 25 provides a substrate contact 24 to an N+ region 26. Windows 27 and 29 provide access to a source contact 28 and a drain contact 30. These contacts 28 and 30 are made through holes 31, 33 which are etched in the underside of the substrate. The sidewalls of the holes are provided with isolating P+ regions 35, 37 for isolating the substrate. The P+ regions 35, 37 are each coated with a metallization 39, 41 to provide electrical connections from the source contact 28 to the source 20 and from the drain contact 30 to the drain 18. The ISFET is then mounted in a probe body such that only a portion of the top surface of the substrate is exposed.
While the ISFET described in the '799 patent is an effective improvement over earlier ISFET devices, the method used to make it is relatively complex. A single transistor is formed on one side of the substrate and two etched holes must be provided on the other side to access the source and drain of the transistor. As with all transistors, the performance of the transistor will be subject to a number of different parameters including ambient temperature. Since pH is also affected by temperature, the reliability of the output of the ISFET will be temperature dependent.